1. Field of the Invention
The present invention relates generally to the field of viruses for vaccine development useful in the prophylaxis of African Swine Fever (ASF). More specifically, the invention provides a novel ASF virus for the development of recombinant ASF virus vaccine candidates, the genomes of which are modified from that of their parental Georgia strain (ASF-G) and are adapted for growth in Vero cells, resulting in the isolated, recombinant ASF-GVAV.
2. Description of the Relevant Art
African swine fever is a devastating highly contagious viral disease of pigs with mortality rates approaching 100 percent. ASF is endemic to Sub-Saharan Africa and maintains a life cycle in the wild through infection between soft ticks and feral pigs (wild pigs/bush pigs/warthogs). It causes major economic losses, threatens food security, and limits pig production in affected countries. The threat for an introduction of ASF in the United States is significant. The appearance of the 2007 outbreak in the Caucasus region (caused by the so called Georgia strain) and its further spreading throughout west Russia and Ukraine indicate that ASF is constant threat not only to Europe but also to Asia where swine represent the main source of animal protein and where the introduction and consequent high mortality caused by ASF would have devastating effects.
The disease occurs in several forms, ranging from acute to chronic with all infections being lethal. Importantly, there is no vaccine available to prevent the disease. Additionally, with the exception of few preliminary studies showing induction of neutralizing antibodies and partial protection against the challenge in animals immunized with a combination of structural proteins, no viral protein(s) mediating protective immunity or the immune mechanisms involved in protection have been identified.
Currently, there is no vaccine available for ASF and disease outbreaks are controlled by animal quarantine and slaughter. African swine fever virus is a large, icosahedral, cytoplasmic, double-stranded DNA virus; it is the only member of the family Asfaviridae, although it shares similarities with other virus families in the superfamily of nucleo-cytoplasmic large DNA viruses (Chapman et al. 2011. Emerging Infect. Dis. 17: 599-605). Attempts to vaccinate animals using infected cell extracts, supernatants of infected pig peripheral blood leukocytes, purified and inactivated virions, infected glutaraldehyde-fixed macrophages, or detergent-treated infected alveolar macrophages failed to induce protective immunity (Coggins, L. 1974. Prog. Med. Virol. 18:48-63; Forman et al. 1982. Arch. Virol. 74:91-100; Kihm et al. 1987. In: African Swine Fever, Becker, Y. (ed), Martinus Nijhoff, Boston, pp 127-144; Mebus, C. A. 1988. Adv. Virus Res. 35:251-269). Conversely, the use of attenuated virus strains obtained either by serial passages in cell cultures or by deleting virulence-associated genes through genetic manipulation of the virus genome constitutes the only methodology to induce protection. Thus, pigs surviving acute infection with moderately virulent or attenuated variants of ASFV develop long-term resistance to homologous, but rarely to heterologous, virus challenge (Hamdy and Dardiri. 1984. Am. J. Vet. Res. 45:711-714; Ruiz-Gonzalvo et al. 1981. In: FAO/CEC Expert Consultation in ASF Research, Wilkinson, P. J. (ed), Rome, pp 206-216). Importantly, pigs immunized with live attenuated ASF viruses containing engineered deletions of specific ASFV virulence/host range genes were protected when challenged with homologous parental virus (Lewis et al. 2000. J. Virol. 74:1275-1285; Moore et al. 1998. J. Virol. 72:10310-10315; Zsak et al. 1996. J. Virol. 70:8865-8871; Zsak et al. 1998. J. Virol. 72:1028-1035). These reports are a proof of concepts regarding the feasibility for the development of ASF vaccines by creating attenuated recombinant virus strains by genetic manipulation of field strains.
The core of the process of producing ASF recombinant viruses includes a homologous recombination event leading to the deletion of a specific virus gene and the insertion of a foreign marker gene which facilitates the identification and further purification of the recombinant virus. The process of developing recombinant ASFV from field isolates is time consuming and requires the availability of primary cultures of swine macrophages. Performing this process using established cell lines would be much easier since cells would be readily available and the process of homologous recombination much more effective. Thus, availability of an ASF virus acclimated to growing in a cell line would be useful and desirable for vaccine development.